Methods and apparatus of video coding using subblock-based temporal motion vector prediction

ABSTRACT

A method of subblock-based temporal motion vector prediction is performed at a computing device. The computing device acquires a video bitstream including data associated with multiple encoded pictures. While decoding a current picture in the video bitstream, the computing device selects, according to syntax elements signaled in the video bitstream, one reference picture as a collocated picture of the current picture, and determines a temporal vector between the collocated picture and the current picture from motion information of spatially neighboring blocks of a current code unit (CU) according to a fixed order. Next, the computing device splits the current CU into multiple sub-CUs, obtains a temporal motion vector predictor for each sub-CU from the temporal vector and motion information of a block in the collocated picture that corresponds to a respective subblock of the current picture and decodes the current CU according to the temporal motion vector predictors.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of InternationalApplication No. PCT/US2019/048864, filed Aug. 29, 2019 which claims thebenefit of U.S. Provisional Application No. 62/724,506, filed Aug. 29,2018, all of which are incorporated herein by reference.

TECHNICAL FIELD

The present application generally relates to video data encoding anddecoding, and in particular, to method and system of video coding usingsubblock-based temporal motion vector prediction.

BACKGROUND

Digital video is supported by a variety of electronic devices, such asdigital televisions, laptop or desktop computers, tablet computers,digital cameras, digital recording devices, digital media players, videogaming consoles, smart phones, video teleconferencing devices, videostreaming devices, etc. The electronic devices transmit, receive,encode, decode, and/or store digital video data by implementing videocompression/decompression standards as defined by MPEG-4, ITU-T H.263,ITU-T H.264/MPEG-4, Part 10, Advanced Video Coding (AVC), HighEfficiency Video Coding (HEVC), and Versatile Video Coding (VVC)standard. Video compression typically includes performing spatial (intraframe) prediction and/or temporal (inter frame) prediction to reduce orremove redundancy inherent in the video data. For block-based videocoding, a video frame is partitioned into one or more slices, each slicehaving multiple video blocks, which may also be referred to as codingtree units (CTUs). Each CTU may contain one coding unit (CU) orrecursively split into smaller CUs until the predefined minimum CU sizeis reached. Each CU (also named leaf CU) contains one or multipletransform units (TUs) and each CU also contains one or multipleprediction units (PUs). Each CU can be coded in either intra, inter orIBC modes. Video blocks in an intra coded (I) slice of a video frame areencoded using spatial prediction with respect to reference samples inneighbor blocks within the same video frame. Video blocks in an intercoded (P or B) slice of a video frame may use spatial prediction withrespect to reference samples in neighbor blocks within the same videoframe or temporal prediction with respect to reference samples in otherprevious and/or future reference video frames.

Spatial or temporal prediction based on a reference block that has beenpreviously encoded, e.g., a neighbor block, results in a predictiveblock for a current video block to be coded. The process of finding thereference block may be accomplished by block matching algorithm.Residual data representing pixel differences between the current blockto be coded and the predictive block is referred to as a residual blockor prediction errors. An inter-coded block is encoded according to amotion vector that points to a reference block in a reference frameforming the predictive block, and the residual block. The process ofdetermining the motion vector is typically referred to as motionestimation. An intra coded block is encoded according to an intraprediction mode and the residual block. For further compression, theresidual block is transformed from the pixel domain to a transformdomain, e.g., frequency domain, resulting in residual transformcoefficients, which may then be quantized. The quantized transformcoefficients, initially arranged in a two-dimensional array, may bescanned to produce a one-dimensional vector of transform coefficients,and then entropy encoded into a video bitstream to achieve even morecompression.

The encoded video bitstream is then saved in a computer-readable storagemedium (e.g., flash memory) to be accessed by another electronic devicewith digital video capability or directly transmitted to the electronicdevice wired or wirelessly. The electronic device then performs videodecompression (which is an opposite process to the video compressiondescribed above) by, e.g., parsing the encoded video bitstream to obtainsyntax elements from the bitstream and reconstructing the digital videodata to its original format from the encoded video bitstream based atleast in part on the syntax elements obtained from the bitstream, andrenders the reconstructed digital video data on a display of theelectronic device.

With digital video quality going from high definition, to 4 K×2 K oreven 8 K×4 K, the amount of vide data to be encoded/decoded growsexponentially. It is a constant challenge in terms of how the video datacan be encoded/decoded more efficiently while maintaining the imagequality of the decoded video data.

SUMMARY

The present application describes implementations related to video dataencoding and decoding and, more particularly, to system and method ofvideo encoding and decoding using subblock-based temporal motion vectorprediction.

According to a first aspect of the present application, a method ofsubblock-based temporal motion vector prediction is performed at acomputing device having one or more processors and memory storing aplurality of programs to be executed by the one or more processors. Thecomputing device acquires a video bitstream including data associatedwith multiple encoded pictures. While decoding a current picture in thevideo bitstream, the computing device selects, according to syntaxelements signaled in the video bitstream, one reference picture as acollocated picture of the current picture, and determines a temporalvector between the collocated picture and the current picture frommotion information of spatially neighboring blocks of a current codeunit (CU) according to a fixed order. Next, the computing device splitsthe current CU into multiple sub-CUs, each sub-CU corresponding to arespective subblock of the current picture. The computing device thenobtains a temporal motion vector predictor for each sub-CU of thecurrent CU from (i) the temporal vector between the collocated pictureand the current picture and (ii) motion information of a block in thecollocated picture that corresponds to the respective subblock of thecurrent picture and decodes the current CU according to the temporalmotion vector predictors of the plurality of sub-CUs of the current CU.

According to a second aspect of the present application, a computingdevice includes one or more processors, memory and a plurality ofprograms stored in the memory. The programs, when executed by the one ormore processors, cause the computing device to perform theaforementioned operations as described above.

According to a third aspect of the present application, a non-transitorycomputer readable storage medium stores a plurality of programs forexecution by a computing device having one or more processors. Theprograms, when executed by the one or more processors, cause thecomputing device to perform the aforementioned operations as describedabove.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the implementations and are incorporated herein andconstitute a part of the specification, illustrate the describedimplementations and together with the description serve to explain theunderlying principles. Like reference numerals refer to correspondingparts.

FIG. 1 is a block diagram illustrating an exemplary video encoding anddecoding system in accordance with some implementations of the presentdisclosure.

FIG. 2 is a block diagram illustrating an exemplary video encoder inaccordance with some implementations of the present disclosure.

FIG. 3 is a block diagram illustrating an exemplary video decoder inaccordance with some implementations of the present disclosure.

FIGS. 4A-4D are block diagrams illustrating how a frame is recursivelyquad-tree partitioned into multiple video blocks of different sizes inaccordance with some implementations of the present disclosure.

FIG. 5A is a block diagram illustrating spatially neighboring andtemporally collocated block positions of a current CU to be encoded inaccordance with some implementations of the present disclosure.

FIG. 5B is a flowchart illustrating an exemplary process by which a listof motion vector candidates are identified in accordance with someimplementations of the present disclosure.

FIG. 5C is a block diagram illustrating how a subblock-based temporalmotion vector prediction is performed between a current picture and acollocated picture in accordance with some implementations of thepresent disclosure.

FIG. 6 is a flowchart illustrating an exemplary process by which a videodecoder implements the techniques of constructing subblock-basedtemporal motion vector prediction for a current picture from motioninformation of a collocated picture in accordance with someimplementations of the present disclosure.

DETAILED DESCRIPTION

Reference will now be made in detail to specific implementations,examples of which are illustrated in the accompanying drawings. In thefollowing detailed description, numerous non-limiting specific detailsare set forth in order to assist in understanding the subject matterpresented herein. But it will be apparent to one of ordinary skill inthe art that various alternatives may be used without departing from thescope of claims and the subject matter may be practiced without thesespecific details. For example, it will be apparent to one of ordinaryskill in the art that the subject matter presented herein can beimplemented on many types of electronic devices with digital videocapabilities.

FIG. 1 is a block diagram illustrating an exemplary system 10 forencoding and decoding video blocks in parallel in accordance with someimplementations of the present disclosure. As shown in FIG. 1, system 10includes a source device 12 that generates and encodes video data to bedecoded at a later time by a destination device 14. Source device 12 anddestination device 14 may comprise any of a wide variety of electronicdevices, including desktop or laptop computers, tablet computers, smartphones, set-top boxes, digital televisions, cameras, display devices,digital media players, video gaming consoles, video streaming device, orthe like. In some implementations, source device 12 and destinationdevice 14 are equipped with wireless communication capabilities.

In some implementations, destination device 14 may receive the encodedvideo data to be decoded via a link 16. Link 16 may comprise any type ofcommunication medium or device capable of moving the encoded video datafrom source device 12 to destination device 14. In one example, link 16may comprise a communication medium to enable source device 12 totransmit the encoded video data directly to destination device 14 inreal-time. The encoded video data may be modulated according to acommunication standard, such as a wireless communication protocol, andtransmitted to destination device 14. The communication medium maycomprise any wireless or wired communication medium, such as a radiofrequency (RF) spectrum or one or more physical transmission lines. Thecommunication medium may form part of a packet-based network, such as alocal area network, a wide-area network, or a global network such as theInternet. The communication medium may include routers, switches, basestations, or any other equipment that may be useful to facilitatecommunication from source device 12 to destination device 14.

In some other implementations, the encoded video data may be transmittedfrom output interface 22 to a storage device 32. Subsequently, theencoded video data in storage device 32 may be accessed by destinationdevice 14 via input interface 28. Storage device 32 may include any of avariety of distributed or locally accessed data storage media such as ahard drive, Blu-ray discs, DVDs, CD-ROMs, flash memory, volatile ornon-volatile memory, or any other suitable digital storage media forstoring encoded video data. In a further example, storage device 32 maycorrespond to a file server or another intermediate storage device thatmay hold the encoded video data generated by source device 12.Destination device 14 may access the stored video data from storagedevice 32 via streaming or downloading. The file server may be any typeof computer capable of storing encoded video data and transmitting theencoded video data to destination device 14. Exemplary file serversinclude a web server (e.g., for a website), an FTP server, networkattached storage (NAS) devices, or a local disk drive. Destinationdevice 14 may access the encoded video data through any standard dataconnection, including a wireless channel (e.g., a Wi-Fi connection), awired connection (e.g., DSL, cable modem, etc.), or a combination ofboth that is suitable for accessing encoded video data stored on a fileserver. The transmission of encoded video data from storage device 32may be a streaming transmission, a download transmission, or acombination of both.

As shown in FIG. 1, source device 12 includes a video source 18, a videoencoder 20 and an output interface 22. Video source 18 may include asource such as a video capture device, e.g., a video camera, a videoarchive containing previously captured video, a video feed interface toreceive video from a video content provider, and/or a computer graphicssystem for generating computer graphics data as the source video, or acombination of such sources. As one example, if video source 18 is avideo camera of a security surveillance system, source device 12 anddestination device 14 may form camera phones or video phones. However,the implementations described in the present application may beapplicable to video coding in general, and may be applied to wirelessand/or wired applications.

The captured, pre-captured, or computer-generated video may be encodedby video encoder 20. The encoded video data may be transmitted directlyto destination device 14 via output interface 22 of source device 12.The encoded video data may also (or alternatively) be stored ontostorage device 32 for later access by destination device 14 or otherdevices, for decoding and/or playback. Output interface 22 may furtherinclude a modem and/or a transmitter.

Destination device 14 includes an input interface 28, a video decoder30, and a display device 34. Input interface 28 may include a receiverand/or a modem and receive the encoded video data over link 16. Theencoded video data communicated over link 16, or provided on storagedevice 32, may include a variety of syntax elements generated by videoencoder 20 for use by video decoder 30 in decoding the video data. Suchsyntax elements may be included within the encoded video datatransmitted on a communication medium, stored on a storage medium, orstored a file server.

In some implementations, destination device 14 may include a displaydevice 34, which can be an integrated display device and an externaldisplay device that is configured to communicate with destination device14. Display device 34 displays the decoded video data to a user, and maycomprise any of a variety of display devices such as a liquid crystaldisplay (LCD), a plasma display, an organic light emitting diode (OLED)display, or another type of display device.

Video encoder 20 and video decoder 30 may operate according toproprietary or industry standards, such as VVC, HEVC, MPEG-4, Part 10,Advanced Video Coding (AVC), or extensions of such standards. It shouldbe understood that the present application is not limited to a specificvideo coding/decoding standard and may be applicable to other videocoding/decoding standards. It is generally contemplated that videoencoder 20 of source device 12 may be configured to encode video dataaccording to any of these current or future standards. Similarly, it isalso generally contemplated that video decoder 30 of destination device14 may be configured to decode video data according to any of thesecurrent or future standards.

Video encoder 20 and video decoder 30 each may be implemented as any ofa variety of suitable encoder circuitry, such as one or moremicroprocessors, digital signal processors (DSPs), application specificintegrated circuits (ASICs), field programmable gate arrays (FPGAs),discrete logic, software, hardware, firmware or any combinationsthereof. When implemented partially in software, an electronic devicemay store instructions for the software in a suitable, non-transitorycomputer-readable medium and execute the instructions in hardware usingone or more processors to perform the video coding/decoding operationsdisclosed in the present disclosure. Each of video encoder 20 and videodecoder 30 may be included in one or more encoders or decoders, eitherof which may be integrated as part of a combined encoder/decoder (CODEC)in a respective device.

FIG. 2 is a block diagram illustrating an exemplary video encoder 20 inaccordance with some implementations described in the presentapplication. Video encoder 20 may perform intra and inter predictivecoding of video blocks within video frames. Intra predictive codingrelies on spatial prediction to reduce or remove spatial redundancy invideo data within a given video frame or picture. Inter predictivecoding relies on temporal prediction to reduce or remove temporalredundancy in video data within adjacent video frames or pictures of avideo sequence.

As shown in FIG. 2, video encoder 20 includes video data memory 40,prediction processing unit 41, decoded picture buffer (DPB) 64, summer50, transform processing unit 52, quantization unit 54, and entropyencoding unit 56. Prediction processing unit 41 further includes motionestimation unit 42, motion compensation unit 44, partition unit 45,intra prediction processing unit 46, and intra block copy (BC) unit 48.In some implementations, video encoder 20 also includes inversequantization unit 58, inverse transform processing unit 60, and summer62 for video block reconstruction. A deblocking filter (not shown) maybe positioned between summer 62 and DPB 64 to filter block boundaries toremove blockiness artifacts from reconstructed video. An in loop filter(not shown) may also be used in addition to the deblocking filter tofilter the output of summer 62. Video encoder 20 may take the form of afixed or programmable hardware unit or may be divided among one or moreof the illustrated fixed or programmable hardware units.

Video data memory 40 may store video data to be encoded by thecomponents of video encoder 20. The video data in video data memory 40may be obtained, for example, from video source 18. DPB 64 is a bufferthat stores reference video data for use in encoding video data by videoencoder 20 (e.g., in intra or inter predictive coding modes). Video datamemory 40 and DPB 64 may be formed by any of a variety of memorydevices. In various examples, video data memory 40 may be on-chip withother components of video encoder 20, or off-chip relative to thosecomponents.

As shown in FIG. 2, after receiving video data, partition unit 45 withinprediction processing unit 41 partitions the video data into videoblocks. This partitioning may also include partitioning a video frameinto slices, tiles, or other larger coding units (CUs) according to apredefined splitting structures such as quad-tree structure associatedwith the video data. The video frame may be divided into multiple videoblocks (or sets of video blocks referred to as tiles). Predictionprocessing unit 41 may select one of a plurality of possible predictivecoding modes, such as one of a plurality of intra predictive codingmodes or one of a plurality of inter predictive coding modes, for thecurrent video block based on error results (e.g., coding rate and thelevel of distortion). Prediction processing unit 41 may provide theresulting intra or inter prediction coded block to summer 50 to generatea residual block and to summer 62 to reconstruct the encoded block foruse as part of a reference frame subsequently. Prediction processingunit 41 also provides syntax elements, such as motion vectors,intra-mode indicators, partition information, and other such syntaxinformation, to entropy encoding unit 56.

In order to select an appropriate intra predictive coding mode for thecurrent video block, intra prediction processing unit 46 withinprediction processing unit 41 may perform intra predictive coding of thecurrent video block relative to one or more neighbor blocks in the sameframe as the current block to be coded to provide spatial prediction.Motion estimation unit 42 and motion compensation unit 44 withinprediction processing unit 41 perform inter predictive coding of thecurrent video block relative to one or more predictive blocks in one ormore reference frames to provide temporal prediction. Video encoder 20may perform multiple coding passes, e.g., to select an appropriatecoding mode for each block of video data.

In some implementations, motion estimation unit 42 determines the interprediction mode for a current video frame by generating a motion vector,which indicates the displacement of a prediction unit (PU) of a videoblock within the current video frame relative to a predictive blockwithin a reference video frame, according to a predetermined patternwithin a sequence of video frames. Motion estimation, performed bymotion estimation unit 42, is the process of generating motion vectors,which estimate motion for video blocks. A motion vector, for example,may indicate the displacement of a PU of a video block within a currentvideo frame or picture relative to a predictive block within a referenceframe (or other coded unit) relative to the current block being codedwithin the current frame (or other coded unit). The predeterminedpattern may designate video frames in the sequence as P frames or Bframes. Intra BC unit 48 may determine vectors, e.g., block vectors, forintra BC coding in a manner similar to the determination of motionvectors by motion estimation unit 42 for inter prediction, or mayutilize motion estimation unit 42 to determine the block vector.

A predictive block is a block of a reference frame that is deemed asclosely matching the PU of the video block to be coded in terms of pixeldifference, which may be determined by sum of absolute difference (SAD),sum of square difference (SSD), or other difference metrics. In someimplementations, video encoder 20 may calculate values for sub-integerpixel positions of reference frames stored in DPB 64. For example, videoencoder 20 may interpolate values of one-quarter pixel positions,one-eighth pixel positions, or other fractional pixel positions of thereference frame. Therefore, motion estimation unit 42 may perform amotion search relative to the full pixel positions and fractional pixelpositions and output a motion vector with fractional pixel precision.

Motion estimation unit 42 calculates a motion vector for a PU of a videoblock in an inter prediction coded frame by comparing the position ofthe PU to the position of a predictive block of a reference frameselected from a first reference frame list (e.g., List0) or a secondreference frame list (e.g., List1), each of which identifies one or morereference frames stored in DPB 64. Motion estimation unit 42 sends thecalculated motion vector to motion compensation unit 44 and then toentropy encoding unit 56.

Motion compensation, performed by motion compensation unit 44, mayinvolve fetching or generating the predictive block based on the motionvector determined by motion estimation unit 42. Upon receiving themotion vector for the PU of the current video block, motion compensationunit 44 may locate a predictive block to which the motion vector pointsin one of the reference frame lists, retrieve the predictive block fromDPB 64, and forward the predictive block to summer 50. Summer 50 thenforms a residual video block of pixel difference values by subtractingpixel values of the predictive block provided by motion compensationunit 44 from the pixel values of the current video block being coded.The pixel difference values forming the residual vide block may includeluma or chroma difference components or both. Motion compensation unit44 may also generate syntax elements associated with the video blocks ofa video frame for use by video decoder 30 in decoding the video blocksof the video frame. The syntax elements may include, for example, syntaxelements defining the motion vector used to identify the predictiveblock, any flags indicating the prediction mode, or any other syntaxinformation described herein. Note that motion estimation unit 42 andmotion compensation unit 44 may be highly integrated, but areillustrated separately for conceptual purposes.

In some implementations, intra BC unit 48 may generate vectors and fetchpredictive blocks in a manner similar to that described above inconnection with motion estimation unit 42 and motion compensation unit44, but with the predictive blocks being in the same frame as thecurrent block being coded and with the vectors being referred to asblock vectors as opposed to motion vectors. In particular, intra BC unit48 may determine an intra-prediction mode to use to encode a currentblock. In some examples, intra BC unit 48 may encode a current blockusing various intra-prediction modes, e.g., during separate encodingpasses, and test their performance through rate-distortion analysis.Next, intra BC unit 48 may select, among the various testedintra-prediction modes, an appropriate intra-prediction mode to use andgenerate an intra-mode indicator accordingly. For example, intra BC unit48 may calculate rate-distortion values using a rate-distortion analysisfor the various tested intra-prediction modes, and select theintra-prediction mode having the best rate-distortion characteristicsamong the tested modes as the appropriate intra-prediction mode to use.Rate-distortion analysis generally determines an amount of distortion(or error) between an encoded block and an original, unencoded blockthat was encoded to produce the encoded block, as well as a bitrate(i.e., a number of bits) used to produce the encoded block. Intra BCunit 48 may calculate ratios from the distortions and rates for thevarious encoded blocks to determine which intra-prediction mode exhibitsthe best rate-distortion value for the block.

In other examples, intra BC unit 48 may use motion estimation unit 42and motion compensation unit 44, in whole or in part, to perform suchfunctions for Intra BC prediction according to the implementationsdescribed herein. In either case, for Intra block copy, a predictiveblock may be a block that is deemed as closely matching the block to becoded, in terms of pixel difference, which may be determined by sum ofabsolute difference (SAD), sum of squared difference (SSD), or otherdifference metrics, and identification of the predictive block mayinclude calculation of values for sub-integer pixel positions.

Whether the predictive block is from the same frame according to intraprediction, or a different frame according to inter prediction, videoencoder 20 may form a residual video block by subtracting pixel valuesof the predictive block from the pixel values of the current video blockbeing coded, forming pixel difference values. The pixel differencevalues forming the residual video block may include both luma and chromacomponent differences.

Intra prediction processing unit 46 may intra-predict a current videoblock, as an alternative to the inter-prediction performed by motionestimation unit 42 and motion compensation unit 44, or the intra blockcopy prediction performed by intra BC unit 48, as described above. Inparticular, intra prediction processing unit 46 may determine an intraprediction mode to use to encode a current block. To do so, intraprediction processing unit 46 may encode a current block using variousintra prediction modes, e.g., during separate encoding passes, and intraprediction processing unit 46 (or a mode select unit, in some examples)may select an appropriate intra prediction mode to use from the testedintra prediction modes. Intra prediction processing unit 46 may provideinformation indicative of the selected intra-prediction mode for theblock to entropy encoding unit 56. Entropy encoding unit 56 may encodethe information indicating the selected intra-prediction mode in thebitstream.

After prediction processing unit 41 determines the predictive block forthe current video block via either inter prediction or intra prediction,summer 50 forms a residual video block by subtracting the predictiveblock from the current video block. The residual video data in theresidual block may be included in one or more transform units (TUs) andis provided to transform processing unit 52. Transform processing unit52 transforms the residual video data into residual transformcoefficients using a transform, such as a discrete cosine transform(DCT) or a conceptually similar transform.

Transform processing unit 52 may send the resulting transformcoefficients to quantization unit 54. Quantization unit 54 quantizes thetransform coefficients to further reduce bit rate. The quantizationprocess may also reduce the bit depth associated with some or all of thecoefficients. The degree of quantization may be modified by adjusting aquantization parameter. In some examples, quantization unit 54 may thenperform a scan of a matrix including the quantized transformcoefficients. Alternatively, entropy encoding unit 56 may perform thescan.

Following quantization, entropy encoding unit 56 entropy encodes thequantized transform coefficients into a video bitstream using, e.g.,context adaptive variable length coding (CAVLC), context adaptive binaryarithmetic coding (CABAC), syntax-based context-adaptive binaryarithmetic coding (SBAC), probability interval partitioning entropy(PIPE) coding or another entropy encoding methodology or technique. Theencoded bitstream may then be transmitted to video decoder 30, orarchived in storage device 32 for later transmission to or retrieval byvideo decoder 30. Entropy encoding unit 56 may also entropy encode themotion vectors and the other syntax elements for the current video framebeing coded.

Inverse quantization unit 58 and inverse transform processing unit 60apply inverse quantization and inverse transformation, respectively, toreconstruct the residual video block in the pixel domain for generatinga reference block for prediction of other video blocks. As noted above,motion compensation unit 44 may generate a motion compensated predictiveblock from one or more reference blocks of the frames stored in DPB 64.Motion compensation unit 44 may also apply one or more interpolationfilters to the predictive block to calculate sub-integer pixel valuesfor use in motion estimation.

Summer 62 adds the reconstructed residual block to the motioncompensated predictive block produced by motion compensation unit 44 toproduce a reference block for storage in DPB 64. The reference block maythen be used by intra BC unit 48, motion estimation unit 42 and motioncompensation unit 44 as a predictive block to inter predict anothervideo block in a subsequent video frame.

FIG. 3 is a block diagram illustrating an exemplary video decoder 30 inaccordance with some implementations of the present application. Videodecoder 30 includes video data memory 79, entropy decoding unit 80,prediction processing unit 81, inverse quantization unit 86, inversetransform processing unit 88, summer 90, and DPB 92. Predictionprocessing unit 81 further includes motion compensation unit 82, intraprediction processing unit 84, and intra BC unit 85. Video decoder 30may perform a decoding process generally reciprocal to the encodingprocess described above with respect to video encoder 20 in connectionwith FIG. 2. For example, motion compensation unit 82 may generateprediction data based on motion vectors received from entropy decodingunit 80, while intra-prediction unit 84 may generate prediction databased on intra-prediction mode indicators received from entropy decodingunit 80.

In some examples, a unit of video decoder 30 may be tasked to performthe implementations of the present application. Also, in some examples,the implementations of the present disclosure may be divided among oneor more of the units of video decoder 30. For example, intra BC unit 85may perform the implementations of the present application, alone, or incombination with other units of video decoder 30, such as motioncompensation unit 82, intra prediction processing unit 84, and entropydecoding unit 80. In some examples, video decoder 30 may not includeintra BC unit 85 and the functionality of intra BC unit 85 may beperformed by other components of prediction processing unit 81, such asmotion compensation unit 82.

Video data memory 79 may store video data, such as an encoded videobitstream, to be decoded by the other components of video decoder 30.The video data stored in video data memory 79 may be obtained, forexample, from storage device 32, from a local video source, such as acamera, via wired or wireless network communication of video data, or byaccessing physical data storage media (e.g., a flash drive or harddisk). Video data memory 79 may include a coded picture buffer (CPB)that stores encoded video data from an encoded video bitstream. Decodedpicture buffer (DPB) 92 of video decoder 30 stores reference video datafor use in decoding video data by video decoder 30 (e.g., in intra orinter predictive coding modes). Video data memory 79 and DPB 92 may beformed by any of a variety of memory devices, such as dynamic randomaccess memory (DRAM), including synchronous DRAM (SDRAM),magneto-resistive RAM (MRAM), resistive RAM (RRAM), or other types ofmemory devices. For illustrative purpose, video data memory 79 and DPB92 are depicted as two distinct components of video decoder 30 in FIG.3. But it will be apparent to one skilled in the art that video datamemory 79 and DPB 92 may be provided by the same memory device orseparate memory devices. In some examples, video data memory 79 may beon-chip with other components of video decoder 30, or off-chip relativeto those components.

During the decoding process, video decoder 30 receives an encoded videobitstream that represents video blocks of an encoded video frame andassociated syntax elements. Video decoder 30 may receive the syntaxelements at the video frame level and/or the video block level. Entropydecoding unit 80 of video decoder 30 entropy decodes the bitstream togenerate quantized coefficients, motion vectors or intra-prediction modeindicators, and other syntax elements. Entropy decoding unit 80 thenforwards the motion vectors and other syntax elements to predictionprocessing unit 81.

When the video frame is coded as an intra predictive coded (I) frame orfor intra coded predictive blocks in other types of frames, intraprediction processing unit 84 of prediction processing unit 81 maygenerate prediction data for a video block of the current video framebased on a signaled intra prediction mode and reference data frompreviously decoded blocks of the current frame.

When the video frame is coded as an inter-predictive coded (i.e., B orP) frame, motion compensation unit 82 of prediction processing unit 81produces one or more predictive blocks for a video block of the currentvideo frame based on the motion vectors and other syntax elementsreceived from entropy decoding unit 80. Each of the predictive blocksmay be produced from a reference frame within one of the reference framelists. Video decoder 30 may construct the reference frame lists, e.g.,List0 and List1, using default construction techniques based onreference frames stored in DPB 92.

In some examples, when the video block is coded according to the intraBC mode described herein, intra BC unit 85 of prediction processing unit81 produces predictive blocks for the current video block based on blockvectors and other syntax elements received from entropy decoding unit80. The predictive blocks may be within a reconstructed region of thesame picture as the current video block defined by video encoder 20.

Motion compensation unit 82 and/or intra BC unit 85 determinesprediction information for a video block of the current video frame byparsing the motion vectors and other syntax elements, and then uses theprediction information to produce the predictive blocks for the currentvideo block being decoded. For example, motion compensation unit 82 usessome of the received syntax elements to determine a prediction mode(e.g., intra or inter prediction) used to code video blocks of the videoframe, an inter prediction frame type (e.g., B or P), constructioninformation for one or more of the reference frame lists for the frame,motion vectors for each inter predictive encoded video block of theframe, inter prediction status for each inter predictive coded videoblock of the frame, and other information to decode the video blocks inthe current video frame.

Similarly, intra BC unit 85 may use some of the received syntaxelements, e.g., a flag, to determine that the current video block waspredicted using the intra BC mode, construction information of whichvideo blocks of the frame are within the reconstructed region and shouldbe stored in DPB 92, block vectors for each intra BC predicted videoblock of the frame, intra BC prediction status for each intra BCpredicted video block of the frame, and other information to decode thevideo blocks in the current video frame.

Motion compensation unit 82 may also perform interpolation using theinterpolation filters as used by video encoder 20 during encoding of thevideo blocks to calculate interpolated values for sub-integer pixels ofreference blocks. In this case, motion compensation unit 82 maydetermine the interpolation filters used by video encoder 20 from thereceived syntax elements and use the interpolation filters to producepredictive blocks.

Inverse quantization unit 86 inverse quantizes the quantized transformcoefficients provided in the bitstream and entropy decoded by entropydecoding unit 80 using the same quantization parameter calculated byvideo encoder 20 for each video block in the video frame to determine adegree of quantization. Inverse transform processing unit 88 applies aninverse transform, e.g., an inverse DCT, an inverse integer transform,or a conceptually similar inverse transform process, to the transformcoefficients in order to reconstruct the residual blocks in the pixeldomain.

After motion compensation unit 82 or intra BC unit 85 generates thepredictive block for the current video block based on the vectors andother syntax elements, summer 90 reconstructs decoded video block forthe current video block by summing the residual block from inversetransform processing unit 88 and a corresponding predictive blockgenerated by motion compensation unit 82 and intra BC unit 85. Anin-loop filter (not pictured) may be positioned between summer 90 andDPB 92 to further process the decoded video block. The decoded videoblocks in a given frame are then stored in DPB 92, which storesreference frames used for subsequent motion compensation of next videoblocks. DPB 92, or a memory device separate from DPB 92, may also storedecoded video for later presentation on a display device, such asdisplay device 34 of FIG. 1.

In a typical video coding process, a video sequence typically includesan ordered set of frames or pictures. Each frame may include threesample arrays, denoted SL, SCb, and SCr. SL is a two-dimensional arrayof luma samples. SCb is a two-dimensional array of Cb chroma samples.SCr is a two-dimensional array of Cr chroma samples. In other instances,a frame may be monochrome and therefore includes only onetwo-dimensional array of luma samples.

As shown in FIG. 4A, video encoder 20 (or more specifically partitionunit 45) generates an encoded representation of a frame by firstpartitioning the frame into a set of coding tree units (CTUs). A videoframe may include an integer number of CTUs ordered consecutively in araster scan order from left to right and from top to bottom. Each CTU isa largest logical coding unit and the width and height of the CTU aresignaled by the video encoder 20 in a sequence parameter set, such thatall the CTUs in a video sequence have the same size being one of128×128, 64×64, 32×32, and 16×16. But it should be noted that thepresent application is not necessarily limited to a particular size. Asshown in FIG. 4B, each CTU may comprise one coding tree block (CTB) ofluma samples, two corresponding coding tree blocks of chroma samples,and syntax elements used to code the samples of the coding tree blocks.The syntax elements describe properties of different types of units of acoded block of pixels and how the video sequence can be reconstructed atthe video decoder 30, including inter or intra prediction, intraprediction mode, motion vectors, and other parameters. In monochromepictures or pictures having three separate color planes, a CTU maycomprise a single coding tree block and syntax elements used to code thesamples of the coding tree block. A coding tree block may be an N×Nblock of samples.

To achieve a better performance, video encoder 20 may recursivelyperform tree partitioning such as binary-tree partitioning, quad-treepartitioning or a combination of both on the coding tree blocks of theCTU and divide the CTU into smaller coding units (CUs). As depicted inFIG. 4C, the 64×64 CTU 400 is first divided into four smaller CU, eachhaving a block size of 32×32. Among the four smaller CUs, CU 410 and CU420 are each divided into four CUs of 16×16 by block size. The two 16×16CUs 430 and 440 are each further divided into four CUs of 8×8 by blocksize. FIG. 4D depicts a quad-tree data structure illustrating the endresult of the partition process of the CTU 400 as depicted in FIG. 4C,each leaf node of the quad-tree corresponding to one CU of a respectivesize ranging from 32×32 to 8×8. Like the CTU depicted in FIG. 4B, eachCU may comprise a coding block (CB) of luma samples and twocorresponding coding blocks of chroma samples of a frame of the samesize, and syntax elements used to code the samples of the coding blocks.In monochrome pictures or pictures having three separate color planes, aCU may comprise a single coding block and syntax structures used to codethe samples of the coding block.

In some implementations, video encoder 20 may further partition a codingblock of a CU into one or more M×N prediction blocks (PB). A predictionblock is a rectangular (square or non-square) block of samples on whichthe same prediction, inter or intra, is applied. A prediction unit (PU)of a CU may comprise a prediction block of luma samples, twocorresponding prediction blocks of chroma samples, and syntax elementsused to predict the prediction blocks. In monochrome pictures orpictures having three separate color planes, a PU may comprise a singleprediction block and syntax structures used to predict the predictionblock. Video encoder 20 may generate predictive luma, Cb, and Cr blocksfor luma, Cb, and Cr prediction blocks of each PU of the CU.

Video encoder 20 may use intra prediction or inter prediction togenerate the predictive blocks for a PU. If video encoder 20 uses intraprediction to generate the predictive blocks of a PU, video encoder 20may generate the predictive blocks of the PU based on decoded samples ofthe frame associated with the PU. If video encoder 20 uses interprediction to generate the predictive blocks of a PU, video encoder 20may generate the predictive blocks of the PU based on decoded samples ofone or more frames other than the frame associated with the PU.

After video encoder 20 generates predictive luma, Cb, and Cr blocks forone or more PUs of a CU, video encoder 20 may generate a luma residualblock for the CU by subtracting the CU's predictive luma blocks from itsoriginal luma coding block such that each sample in the CU's lumaresidual block indicates a difference between a luma sample in one ofthe CU's predictive luma blocks and a corresponding sample in the CU'soriginal luma coding block. Similarly, video encoder 20 may generate aCb residual block and a Cr residual block for the CU, respectively, suchthat each sample in the CU's Cb residual block indicates a differencebetween a Cb sample in one of the CU's predictive Cb blocks and acorresponding sample in the CU's original Cb coding block and eachsample in the CU's Cr residual block may indicate a difference between aCr sample in one of the CU's predictive Cr blocks and a correspondingsample in the CU's original Cr coding block.

Furthermore, as illustrated in FIG. 4C, video encoder 20 may usequad-tree partitioning to decompose the luma, Cb, and Cr residual blocksof a CU into one or more luma, Cb, and Cr transform blocks. A transformblock is a rectangular (square or non-square) block of samples on whichthe same transform is applied. A transform unit (TU) of a CU maycomprise a transform block of luma samples, two corresponding transformblocks of chroma samples, and syntax elements used to transform thetransform block samples. Thus, each TU of a CU may be associated with aluma transform block, a Cb transform block, and a Cr transform block. Insome examples, the luma transform block associated with the TU may be asub-block of the CU's luma residual block. The Cb transform block may bea sub-block of the CU's Cb residual block. The Cr transform block may bea sub-block of the CU's Cr residual block. In monochrome pictures orpictures having three separate color planes, a TU may comprise a singletransform block and syntax structures used to transform the samples ofthe transform block.

Video encoder 20 may apply one or more transforms to a luma transformblock of a TU to generate a luma coefficient block for the TU. Acoefficient block may be a two-dimensional array of transformcoefficients. A transform coefficient may be a scalar quantity. Videoencoder 20 may apply one or more transforms to a Cb transform block of aTU to generate a Cb coefficient block for the TU. Video encoder 20 mayapply one or more transforms to a Cr transform block of a TU to generatea Cr coefficient block for the TU.

After generating a coefficient block (e.g., a luma coefficient block, aCb coefficient block or a Cr coefficient block), video encoder 20 mayquantize the coefficient block. Quantization generally refers to aprocess in which transform coefficients are quantized to possibly reducethe amount of data used to represent the transform coefficients,providing further compression. After video encoder 20 quantizes acoefficient block, video encoder 20 may entropy encode syntax elementsindicating the quantized transform coefficients. For example, videoencoder 20 may perform Context-Adaptive Binary Arithmetic Coding (CABAC)on the syntax elements indicating the quantized transform coefficients.Finally, video encoder 20 may output a bitstream that includes asequence of bits that forms a representation of coded frames andassociated data, which is either saved in storage device 32 ortransmitted to destination device 14.

After receiving a bitstream generated by video encoder 20, video decoder30 may parse the bitstream to obtain syntax elements from the bitstream.Video decoder 30 may reconstruct the frames of the video data based atleast in part on the syntax elements obtained from the bitstream. Theprocess of reconstructing the video data is generally reciprocal to theencoding process performed by video encoder 20. For example, videodecoder 30 may perform inverse transforms on the coefficient blocksassociated with TUs of a current CU to reconstruct residual blocksassociated with the TUs of the current CU. Video decoder 30 alsoreconstructs the coding blocks of the current CU by adding the samplesof the predictive blocks for PUs of the current CU to correspondingsamples of the transform blocks of the TUs of the current CU. Afterreconstructing the coding blocks for each CU of a frame, video decoder30 may reconstruct the frame.

As noted above, video coding achieves video compression using primarilytwo modes, i.e., intra-frame prediction (or intra-prediction) andinter-frame prediction (or inter-prediction). It is noted that IBC couldbe regarded as either intra-frame prediction or a third mode. Betweenthe two modes, inter-frame prediction contributes more to the codingefficiency than intra-frame prediction because of the use of motionvectors for predicting a current video block from a reference videoblock.

But with the ever improving video data capturing technology and morerefined video block size for preserving details in the video data, theamount of data required for representing motion vectors for a currentframe also increases substantially. One way of overcoming this challengeis to benefit from the fact that not only a group of neighboring CUs inboth the spatial and temporal domains have similar video data forpredicting purpose but the motion vectors between these neighboring CUsare also similar. Therefore, it is possible to use the motioninformation of spatially neighboring CUs and/or temporally collocatedCUs as an approximation of the motion information (e.g., motion vector)of a current CU by exploring their spatial and temporal correlation,which is also referred to as “motion vector predictor” (MVP) of thecurrent CU.

Instead of encoding, into the video bitstream, an actual motion vectorof the current CU determined by motion estimation unit 42 as describedabove in connection with FIG. 2, the motion vector predictor of thecurrent CU is subtracted from the actual motion vector of the current CUto produce a motion vector difference (MVD) for the current CU. By doingso, there is no need to encode the actual motion vector determined bymotion estimation unit 42 for each CU of a frame into the videobitstream and the amount of data used for representing motioninformation in the video bitstream can be significantly decreased.

Like the process of choosing a predictive block in a reference frameduring inter-frame prediction of a code block, a set of rules need to beadopted by both video encoder 20 and video decoder 30 for constructing amotion vector candidate list for a current CU using those potentialcandidate motion vectors associated with spatially neighboring CUsand/or temporally collocated CUs of the current CU and then selectingone member from the motion vector candidate list as a motion vectorpredictor for the current CU. By doing so, there is no need to transmitthe motion vector candidate list itself between video encoder 20 andvideo decoder 30 and an index of the selected motion vector predictorwithin the motion vector candidate list is sufficient for video encoder20 and video decoder 30 to use the same motion vector predictor withinthe motion vector candidate list for encoding and decoding the currentCU.

In some implementations, each inter-prediction CU has three motionvector prediction modes including inter (which is also referred to as“advanced motion vector prediction” (AMVP)), skip, and merge forconstructing the motion vector candidate list. Under each mode, one ormore motion vector candidates may be added to the motion vectorcandidate list according to the algorithms described below. Ultimatelyone of them in the candidate list is used as the best motion vectorpredictor of the inter-prediction CU to be encoded into the videobitstream by video encoder 20 or decoded from the video bitstream byvideo decoder 30. To find the best motion vector predictor from thecandidate list, a motion vector competition (MVC) scheme is introducedto select a motion vector from a given candidate set of motion vectors,i.e., the motion vector candidate list, that includes spatial andtemporal motion vector candidates.

After one MVP candidate is selected within the given candidate set ofmotion vectors for a current CU, video encoder 20 may generate one ormore syntax elements for the corresponding MVP candidate and encode theminto the video bitstream such that video decoder 30 can retrieve the MVPcandidate from the video bitstream using the syntax elements. Dependingon the specific mode used for constructing the motion vectors candidateset, different modes (e.g., AMVP, merge, skip, etc.) have different setsof syntax elements. For the AMVP mode, the syntax elements include interprediction indicators (e.g., List0, List1, or bi-directionalprediction), reference indices, motion vector candidate indices, motionvector differences and prediction residual signal, etc. For the skipmode and the merge mode, only merge indices are encoded into thebitstream because the current CU inherits the other syntax elementsincluding the inter prediction indicators, reference indices, and motionvectors from a neighboring CU referred by the coded merge index. In thecase of a skip coded CU, the motion vector prediction residual signal isalso omitted.

FIG. 5A is a block diagram illustrating spatially neighboring andtemporally collocated block positions of a current CU to beencoded/decoded in accordance with some implementations of the presentdisclosure. For a given mode (e.g., AMVP, merge, or skip), a motionvector prediction (MVP) candidate list is constructed by first checkingthe availability of motion vectors associated with the spatially left(A0, A1) and above (B0, B1, B2) neighboring block positions, and theavailability of motion vectors associated with temporally collocatedblock positions. During the process of constructing the MVP candidatelist, redundant MVP candidates are removed from the candidate list and,if necessary, zero-valued motion vector is added to make the candidatelist to have a fixed length (note that different modes may havedifferent fixed lengths). After the construction of the MVP candidatelist, video encoder 20 can select the best motion vector predictor fromthe candidate list and encode the corresponding index indicating thechosen candidate into the video bitstream.

Using FIG. 5A as an example and assuming that the candidate list has afixed length of two, FIG. 5B is a flowchart illustrating an exemplaryprocess by which a list of motion vector candidates are identified inaccordance with some implementations of the present disclosure. Inparticular, the motion vector predictor (MVP) candidate list for thecurrent CU may be constructed by performing the following steps underthe AMVP mode as depicted in FIG. 5B:

-   -   1) Step 505: Selection of two MVP candidates from five spatially        neighboring CUs        -   a) Derive up to one non-scaled MVP candidate from one of the            two left spatial neighbour CUs starting with A0 and ending            with A1;        -   b) If no non-scaled MVP candidate from left is available in            the previous step, derive up to one scaled MVP candidate            from one of the two left spatial neighbour CUs starting with            A0 and ending with A1;        -   c) Derive up to one non-scaled MVP candidate from one of the            three above spatial neighbour CUs starting with B0, then B1,            and ending with B2;        -   d) If neither A0 nor A1 is available or if they are coded in            intra modes, derive up to one scaled MVP candidate from one            of the three above spatial neighbour CUs starting with B0,            then B1, and ending with B2;    -   2) Step 510: Selection of one MVP candidate from two temporally        collocated CUs;    -   3) Step 515: Removal of duplicate MVP candidates found in the        previous steps from the MVP candidate list;    -   4) Step 520: Add up to two zero-valued MVPs to the MVP candidate        list.    -   5) Step 525: Removal of MVP candidates whose index is larger        than 1 from the MVP candidate list.    -   6) Step 530: Finalization of the two MVP candidates in the MVP        candidate list for the current CU.

Since there are only two candidates in the AMVP-mode MVP candidate listconstructed above, an associated syntax element like a binary flag isencoded into the bitstream to indicate that which of the two MVPcandidates within the candidate list is used for decoding the currentCU.

In some implementations, the process of selecting a temporal motionvector predictor for encoding/decoding the current CU is performed at asub-CU level so as to improve the accuracy of a decoded picture. Thisprocess first identifies a collocated picture for the current pictureincluding the current CU and then determines a temporal vector (alsoknown as “motion shift” in the present application). Next, the processsplits the current CU into multiple sub-CUs and derives motioninformation for each of the sub-CUs from a corresponding block in thecollocated picture identified by the temporal vector according to apredefined algorithm, which is also referred to as “subblock-basedtemporal motion vector prediction” (SbTMVP).

FIG. 5C is a block diagram illustrating how a subblock-based temporalmotion vector prediction is performed between a current picture and acollocated picture in accordance with some implementations of thepresent disclosure. In this example, the current CU 535-1 is a 64×64code block and it is divided into 8×8 sub-CUs, each sub-CU being an 8×8code block. In order to derive the motion information for each sub-CU,the SbTMVP process is divided into two main steps:

-   -   Step One: Identify the corresponding block 540-1 in the        collocated picture 540 using a so-called “temporal vector”        between the current picture 535 and the collocated picture 540.    -   Step Two: Split the current CU 535-1 into multiple sub-CUs and        obtain motion information for the sub-CUs including the motion        vector and the reference index (which is zero by default) of        each sub-CU 535-3 from a corresponding block in the collocated        picture 540.

As noted above, the collocated picture 540 is assumed to be known forthe current picture 535 before the execution of the SbTMVP process. Forexample, the collocated picture is typically one reference picture ofthe current picture, which is selected from one of the two referencepicture lists of the current picture, e.g., List0 and List1. In someimplementations, the corresponding block is the one at the same relativeposition in the collocated picture as the current CU in the currentpicture. In some other implementations (e.g., FIG. 5C), thecorresponding block is not necessarily the one at the same relativeposition in the collocated picture as the current CU 535-1 in thecurrent picture. Instead, there is a temporal vector linking the centerof the current CU 535-1 in the current picture to the center of thecorresponding block 540-1 in the collocated picture 540.

In some implementations, the SbTMVP process is implemented as part ofthe process of constructing a motion vector candidate list describedabove for a current CU during video encoding. In other words, if it isdetermined to that the current CU is to be processed using the SbTMVP, aparameter corresponding to the SbTMVP is added to the motion vectorcandidate list accordingly. In some other implementations, the SbTMVPprocess is implemented independently from the process of constructing amotion vector candidate list described above for a current CU duringvideo encoding. In other words, the SbTMVP is treated as a separateinter-prediction mode like the aforementioned inter-prediction modes.Due to the symmetric nature between the encoding process and thedecoding process, the rest of the present application uses the decodingof a current CU to illustrate how the SbTMVP process is employed forpredicting the temporal motion vector of the current CU.

FIG. 6 is a flowchart illustrating an exemplary decoding process bywhich a video decoder implements the techniques of constructingsubblock-based temporal motion vector prediction for a current picturefrom motion information of a collocated picture in accordance with someimplementations of the present disclosure.

First, video decoder 30 acquires (610) an encoded video bitstreamincluding data associated with multiple encoded pictures. As depicted inFIGS. 4A and 4C, each picture includes multiple rows of coding treeunits (CTUs) and each CTU includes one or more coding units (CUs). Videodecoder 30 extracts different pieces of information from the videobitstream, such as syntax elements and pixel values, to reconstruct thepicture row by row.

In this example, it is assumed that video decoder 30 is decoding thecurrent picture 535 depicted in FIG. 5C (630) or more specifically, acurrent CU 535-1 in the current picture 535. As noted above, the currentpicture 535 has multiple reference pictures, e.g., List0 and/or List.For the purpose of temporal motion vector prediction, one of thereference pictures is the so-called “collocated picture” 540 of thecurrent picture 535 as depicted in FIG. 5C. Therefore, video decoder 30first determine one of the reference pictures as a collocated picture ofthe current picture according to the syntax elements signaled in thevideo bitstream in a predefined order (630-1). For example, in thederivation of the temporal motion vector candidates, an explicit flag inthe slice header (collocated_from_10_flag) is firstly sent to videodecoder 30 to indicate whether the collocated picture is selected from,e.g., List0 or List1. A collocated reference index (collocated_ref_idx)is further sent to video decoder 30 indicating which reference picturein that list is selected as the collocated picture for deriving thetemporal motion vector candidate.

In either case, after a reference picture of the current picture isidentified as being the collocated picture, video decoder 30 determinesa motion shift (also known as “temporal vector”) between the collocatedpicture and the current picture (see, e.g., temporal vector 537 depictedin FIG. 5C) from the motion information of the spatially neighbouringblocks of the current CU 535-1 according to a fixed order (630-3). Asnoted above, each CTU (including one or more CUs) has multiple spatiallyneighbouring blocks like A0, A1, B0, B1, B2, etc. Each of the spatiallyneighbouring blocks may have a motion vector pointing to a correspondingblock within a respective reference picture of the current CU. In someimplementations, video decoder 30 checks each reference pictureassociated with a corresponding spatially neighbouring block until oneof them is the same as the collocated picture. The checking order isadaptive. In some implementations, the checking order starts with one ofList0 and List1 according to the low delay condition (LDC) and thesyntax element “collocated_from_10_flag”. LDC is a Boolean variable toindicate whether all reference pictures have smaller Picture Order Count(POC) then the current picture. For example, List0 may include at leastone reference picture preceding the current picture in time andoptionally, one or more reference pictures following the current picturein time. List1 may include at least one reference picture following thecurrent picture in time or only reference pictures preceding the currentpicture in time. After identifying the reference picture associated witha particular spatially neighbouring block of the current CU, videodecoder 30 determines the motion information of the spatiallyneighbouring block and uses it as the motion shift between thecollocated picture and the current picture. Using the motion shift, ablock within the collocated picture can be identified as correspondingto the current CU. After establishing a mapping relationship between thecurrent CU 535-1 and the corresponding block 540-1 as depicted in FIG.5C, video decoder 30 can start constructing temporal motion vectorprediction for each subblock of the current CU 535-1.

Note that it is assumed that video encoder 20 has split the current CU535-1 into multiple sub-CUs during the generation of the video bitstreamreceived by video decoder 30. Therefore, video decoder 30 can use, fromthe video bitstream, the same set of syntax elements for splitting thecurrent CU 535-1 into multiple sub-CUs (630-5). As depicted in FIG. 5C,each sub-CU 535-3 in the current picture 535 has a correspondingsubblock at the same relative position in the collocated picture 540.Video decoder 30 is responsible for reconstructing the same temporalmotion vector prediction for each sub-CU as video encoder 20 does whenencoding the sub-CU into the video bitstream according to the SbTMVPprocess.

In some implementations, video decoder 30 obtains a temporal motionvector predictor for each sub-CU of the current CU based on two piecesof information, i.e., the motion shift between the collocated pictureand the current picture and motion information of a block in thecollocated picture that corresponds to the respective subblock of thecurrent picture (630-7). For example, for each sub-CU in the currentpicture 535, video decoder 30 identifies a block in the collocatedpicture at a same relative location as the subblock of the sub-CU in thecurrent picture according to the motion shift between the collocatedpicture and the current picture. Next, video decoder 30 determinesmotion information of the identified block in the collocated picture andselects a motion vector as well as a reference index from the determinedmotion information of the identified block to derive the temporal motionvector predictor of the sub-CU according to the ratio of the POCdifferences between the current picture and its reference picture andthe POC differences between the collocated picture and the referencepicture of the corresponding block.

On the other hand, the corresponding block 540-1 in the collocatedpicture 540 may belong to different CUs, CTUs or even different slicesor tiles. It is possible that different subblocks in the correspondingblock 540-1 may have different prediction modes such that some of thesubblocks in the corresponding block 540-1 may not have motion vectorsat all. In some implementations, video decoder 30 deals with thissituation by checking a particular subblock within the correspondingblock 540-1 and determining whether the particular subblock has motioninformation. If the particular subblock has motion information, videodecoder 30 may use the particular subblock's as a default temporalmotion vector predictor for another sub-CU of the current CU when thesub-CU's corresponding block in the collocated picture does not havemotion information. For example, before processing any sub-CU in thecurrent CU, video decoder 30 may first examine the corresponding blockof a block (or a sample) at or near the center of the correspondingblock 540-1 (e.g., the one at the right side immediately below thecenter of the corresponding block 540-1) and check whether the block hasmotion information or not. If the block has no motion information, videodecoder 30 then assumes that the SbTMVP process does not apply to thecurrent CU and proceeds to process another merge candidates in thecurrent picture. But if the block has motion information, video decoder30 assumes that the SbTMVP process applies to the current CU and usesthe motion information as the default temporal motion vector predictorfor any sub-CU of the current CU when its corresponding block at thesame relative location in the collocated picture does not have motioninformation for constructing the sub-CU's temporal motion vectorpredictor. Assuming that the SbTMVP process applies to the current CU,video decoder 30 uses the temporal motion vector predictors obtained forthe plurality of sub-CUs of the current CU to decode the current CUaccordingly (630-9).

As noted above, intra block copy (IBC) can significantly improve thecoding efficiency of screen content materials. Since IBC mode isimplemented as a block-level coding mode, block matching (BM) isperformed at video encoder 20 to find an optimal block vector for eachCU. Here, a block vector is used to indicate the displacement from thecurrent block to a reference block, which has already been reconstructedwithin the current picture. An IBC-coded CU is treated as the thirdprediction mode other than the intra or inter prediction modes.

At the CU level, the IBC mode can be signaled as IBC AMVP mode or IBCskip/merge mode as follows:

-   -   IBC AMVP mode: a block vector difference (BVD) between the        actual block vector of a CU and a block vector predictor of the        CU selected from block vector candidates of the CU is encoded in        the same way as a motion vector difference is encoded under the        AMVP mode described above. The block vector prediction method        uses two block vector candidates as predictors, one from left        neighbor and the other one from above neighbor (if IBC coded).        When either neighbor is not available, a default block vector        will be used as a block vector predictor. A binary flag is        signaled to indicate the block vector predictor index.    -   IBC skip/merge mode: a merge candidate index is used to indicate        which of the block vector candidates in the merge candidate list        from neighboring IBC coded blocks is used to predict the block        vector for the current block.

The video coder may initialize a context for a current wavefront forperforming context adaptive binary arithmetic coding (CABAC) of thecurrent wavefront based on data of the first two blocks of the abovewavefront, as well as one or more elements of a slice header for a sliceincluding the first code block of the current wavefront. The video codermay perform CABAC initialization of a subsequent wavefront (or CTU row)using the context states after coding two CTUs of a CTU row above thesubsequent CTU row. In other words, before beginning coding of a currentwavefront, a video coder may code at least two blocks of a wavefrontabove the current wavefront, assuming the current wavefront is not thetop row of CTUs of a picture. The video coder may then initialize aCABAC context for the current wavefront after coding at least two blocksof a wavefront above the current wavefront.

In one or more examples, the functions described may be implemented inhardware, software, firmware, or any combination thereof. If implementedin software, the functions may be stored on or transmitted over, as oneor more instructions or code, a computer-readable medium and executed bya hardware-based processing unit. Computer-readable media may includecomputer-readable storage media, which corresponds to a tangible mediumsuch as data storage media, or communication media including any mediumthat facilitates transfer of a computer program from one place toanother, e.g., according to a communication protocol. In this manner,computer-readable media generally may correspond to (1) tangiblecomputer-readable storage media which is non-transitory or (2) acommunication medium such as a signal or carrier wave. Data storagemedia may be any available media that can be accessed by one or morecomputers or one or more processors to retrieve instructions, codeand/or data structures for implementation of the implementationsdescribed in the present application. A computer program product mayinclude a computer-readable medium.

The terminology used in the description of the implementations herein isfor the purpose of describing particular implementations only and is notintended to limit the scope of claims. As used in the description of theimplementations and the appended claims, the singular forms “a,” “an,”and “the” are intended to include the plural forms as well, unless thecontext clearly indicates otherwise. It will also be understood that theterm “and/or” as used herein refers to and encompasses any and allpossible combinations of one or more of the associated listed items. Itwill be further understood that the terms “comprises” and/or“comprising,” when used in this specification, specify the presence ofstated features, elements, and/or components, but do not preclude thepresence or addition of one or more other features, elements,components, and/or groups thereof.

It will also be understood that, although the terms first, second, etc.may be used herein to describe various elements, these elements shouldnot be limited by these terms. These terms are only used to distinguishone element from another. For example, a first electrode could be termeda second electrode, and, similarly, a second electrode could be termed afirst electrode, without departing from the scope of theimplementations. The first electrode and the second electrode are bothelectrodes, but they are not the same electrode.

The description of the present application has been presented forpurposes of illustration and description, and is not intended to beexhaustive or limited to the invention in the form disclosed. Manymodifications, variations, and alternative implementations will beapparent to those of ordinary skill in the art having the benefit of theteachings presented in the foregoing descriptions and the associateddrawings. The embodiment was chosen and described in order to bestexplain the principles of the invention, the practical application, andto enable others skilled in the art to understand the invention forvarious implementations and to best utilize the underlying principlesand various implementations with various modifications as are suited tothe particular use contemplated. Therefore, it is to be understood thatthe scope of claims is not to be limited to the specific examples of theimplementations disclosed and that modifications and otherimplementations are intended to be included within the scope of theappended claims.

What is claimed is:
 1. A method of subblock-based temporal motion vectorprediction, the method comprising: acquiring a video bitstream includingdata associated with multiple encoded pictures, each picture including aplurality of coding units (CUs); while decoding a current picture of thevideo bitstream, the current picture having a plurality of referencepictures: selecting, according to syntax elements signaled in the videobitstream, one of the reference pictures as a collocated picture of thecurrent picture; determining a temporal vector between the collocatedpicture and the current picture by (i) checking reference picturesassociated with spatially neighbouring blocks of a current CU in thecurrent picture according to a fixed order until one reference pictureassociated with one of the spatially neighbouring blocks is the same asthe collocated picture and (ii) choosing a motion vector of thespatially neighbouring block pointing to the reference picture as thetemporal vector for the current CU; splitting the current CU into aplurality of sub-CUs, each sub-CU corresponding to a respective subblockof the current picture; obtaining a temporal motion vector predictor foreach sub-CU of the current CU from (i) the temporal vector between thecollocated picture and the current picture and (ii) motion informationof a block in the collocated picture that corresponds to the respectivesubblock of the current picture; and decoding the current CU accordingto the temporal motion vector predictors of the plurality of sub-CUs ofthe current CU.
 2. The method of claim 1, wherein the plurality ofreference pictures includes a first list of reference pictures and asecond list of reference pictures distinct from the first list ofreference pictures, the first list including at least one referencepicture preceding the current picture in time and the second listincluding at least one reference picture following the current picturein time.
 3. The method of claim 1, wherein the plurality of referencepictures includes a first list of reference pictures and a second listof reference pictures, the fixed order is a predefined order of theplurality of reference pictures, and the predefined order is to processthe first list of reference pictures before processing the second listof reference pictures.
 4. The method of claim 1, wherein the pluralityof reference pictures includes a first list of reference pictures and asecond list of reference pictures, the fixed order is a predefined orderof the plurality of reference pictures, and the predefined order is toprocess the second list of reference pictures before processing thefirst list of reference pictures.
 5. The method of claim 1, wherein theobtaining a temporal motion vector predictor for each sub-CU of thecurrent CU further comprises: identifying a block in the collocatedpicture at a same relative location as the subblock of the sub-CU in thecurrent picture according to the temporal vector between the collocatedpicture and the current picture; determining motion information of theidentified block in the collocated picture; and deriving a motion vectorfrom the determined motion information of the identified block as thetemporal motion vector predictor of the sub-CU.
 6. The method of claim1, wherein the obtaining a temporal motion vector predictor for eachsub-CU of the current CU further comprises: identifying a block in thecollocated picture corresponding to a subblock of a sub-CU at or closeto a center of the current CU in the current picture; determining motioninformation of the identified block in the collocated picture; andderiving a motion vector from the determined motion information of theidentified block as a default temporal motion vector predictor of anysub-CU of the current CU whose corresponding block in the collocatedpicture does not have motion information.
 7. A computing devicecomprising: one or more processors; memory coupled to the one or moreprocessors; and a plurality of programs stored in the memory that, whenexecuted by the one or more processors, cause the computing device toperform operations including: acquiring a video bitstream including dataassociated with multiple encoded pictures, each picture including aplurality of coding units (CUs); while decoding a current picture of thevideo bitstream, the current picture having a plurality of referencepictures: selecting, according to syntax elements signaled in the videobitstream, one of the reference pictures as a collocated picture of thecurrent picture; determining a temporal vector between the collocatedpicture and the current picture by (i) checking reference picturesassociated with spatially neighbouring blocks of a current CU in thecurrent picture according to a fixed order until one reference pictureassociated with one of the spatially neighbouring blocks is the same asthe collocated picture and (ii) choosing a motion vector of thespatially neighbouring block pointing to the reference picture as thetemporal vector for the current CU; splitting the current CU into aplurality of sub-CUs, each sub-CU corresponding to a respective subblockof the current picture; obtaining a temporal motion vector predictor foreach sub-CU of the current CU from (i) the temporal vector between thecollocated picture and the current picture and (ii) motion informationof a block in the collocated picture that corresponds to the respectivesubblock of the current picture; and decoding the current CU accordingto the temporal motion vector predictors of the plurality of sub-CUs ofthe current CU.
 8. The computing device of claim 7, wherein theplurality of reference pictures includes a first list of referencepictures and a second list of reference pictures distinct from the firstlist of reference pictures, the first list including at least onereference picture preceding the current picture in time and the secondlist including at least one reference picture following the currentpicture in time.
 9. The computing device of claim 7, wherein theplurality of reference pictures includes a first list of referencepictures and a second list of reference pictures, the fixed order is apredefined order of the plurality of reference pictures, and thepredefined order is to process the first list of reference picturesbefore processing the second list of reference pictures.
 10. Thecomputing device of claim 7, wherein the plurality of reference picturesincludes a first list of reference pictures and a second list ofreference pictures, the fixed order is a predefined order of theplurality of reference pictures, and the predefined order is to processthe second list of reference pictures before processing the first listof reference pictures.
 11. The computing device of claim 7, wherein theobtaining a temporal motion vector predictor for each sub-CU of thecurrent CU further comprises: identifying a block in the collocatedpicture at a same relative location as the subblock of the sub-CU in thecurrent picture according to the temporal vector between the collocatedpicture and the current picture; determining motion information of theidentified block in the collocated picture; and deriving a motion vectorfrom the determined motion information of the identified block as thetemporal motion vector predictor of the sub-CU.
 12. The computing deviceof claim 7, wherein the obtaining a temporal motion vector predictor foreach sub-CU of the current CU further comprises: identifying a block inthe collocated picture corresponding to a subblock of a sub-CU at orclose to a center of the current CU in the current picture; determiningmotion information of the identified block in the collocated picture;and deriving a motion vector from the determined motion information ofthe identified block as a default temporal motion vector predictor ofany sub-CU of the current CU whose corresponding block in the collocatedpicture does not have motion information.
 13. A non-transitory computerreadable storage medium storing a plurality of programs for execution bya computing device having one or more processors, wherein the pluralityof programs, when executed by the one or more processors, cause thecomputing device to perform operations including: acquiring a videobitstream including data associated with multiple encoded pictures, eachpicture including a plurality of coding units (CUs); while decoding acurrent picture of the video bitstream, the current picture having aplurality of reference pictures: selecting, according to syntax elementssignaled in the video bitstream, one of the reference pictures as acollocated picture of the current picture; selecting, according tosyntax elements signaled in the video bitstream, one of the referencepictures as a collocated picture of the current picture; determining atemporal vector between the collocated picture and the current pictureby (i) checking reference pictures associated with spatiallyneighbouring blocks of a current CU in the current picture according toa fixed order until one reference picture associated with one of thespatially neighbouring blocks is the same as the collocated picture and(ii) choosing a motion vector of the spatially neighbouring blockpointing to the reference picture as the temporal vector for the currentCU; splitting the current CU into a plurality of sub-CUs, each sub-CUcorresponding to a respective subblock of the current picture; obtaininga temporal motion vector predictor for each sub-CU of the current CUfrom (i) the temporal vector between the collocated picture and thecurrent picture and (ii) motion information of a block in the collocatedpicture that corresponds to the respective subblock of the currentpicture; and decoding the current CU according to the temporal motionvector predictors of the plurality of sub-CUs of the current CU.
 14. Thenon-transitory computer readable storage medium of claim 13, wherein theplurality of reference pictures includes a first list of referencepictures and a second list of reference pictures distinct from the firstlist of reference pictures, the first list including at least onereference picture preceding the current picture in time and the secondlist including at least one reference picture following the currentpicture in time.
 15. The non-transitory computer readable storage mediumof claim 13, wherein the plurality of reference pictures includes afirst list of reference pictures and a second list of referencepictures, the fixed order is a predefined order of the plurality ofreference pictures, and the predefined order is to process the firstlist of reference pictures before processing the second list ofreference pictures.
 16. The non-transitory computer readable storagemedium of claim 13, wherein the plurality of reference pictures includesa first list of reference pictures and a second list of referencepictures, the fixed order is a predefined order of the plurality ofreference pictures, and the predefined order is to process the secondlist of reference pictures before processing the first list of referencepictures.
 17. The non-transitory computer readable storage medium ofclaim 13, wherein the obtaining a temporal motion vector predictor foreach sub-CU of the current CU further comprises: identifying a block inthe collocated picture at a same relative location as the subblock ofthe sub-CU in the current picture according to the temporal vectorbetween the collocated picture and the current picture; determiningmotion information of the identified block in the collocated picture;and deriving a motion vector from the determined motion information ofthe identified block as the temporal motion vector predictor of thesub-CU, or wherein the obtaining a temporal motion vector predictor foreach sub-CU of the current CU further comprises: identifying a block inthe collocated picture corresponding to a subblock of a sub-CU at orclose to a center of the current CU in the current picture; determiningmotion information of the identified block in the collocated picture;and deriving a motion vector from the determined motion information ofthe identified block as a default temporal motion vector predictor ofany sub-CU of the current CU whose corresponding block in the collocatedpicture does not have motion information.